Assuming the camera meter says you're properly exposed for the ISO 100 shot, it'll say you're one stop underexposed for the ISO 50 shot and one stop overexposed for the ISO 200 shot. But, if you ignore the meter and use the same shutter and aperture for all three shots, only changing ISO, you will get the exact same raw file.

...again, with the caveat that the metadata will indicate the ISO you had the camera set to, which will result in one stop of digital (i.e in ACR or DPP or LR or wherever) underexposure for the ISO 50 shot and one stop of digital overexposure for the ISO 200 + HTP shot. That means that, though the raw files are identical for all three, the numerical values recorded in them are divided by 2 for ISO 50 and multiplied by 2 for ISO 200 + HTP before any other processing is done. (It's a linear operation for ISO 50 and a non-linear graduated operation for ISO 200 + HTP.

In contrast, if you were to make the exact same exposure but with ISO 200 without HTP, analog electrical amplification would be applied to the signal before the analog-to-digital converter reads the data, sufficient to double the signal strength. The actual data recorded to the raw file would be different, though it would bear a superficial resemblance to the HTP shot after the initial digital compensation had been applied. Indeed, the regular, non-HTP shot would have more useful dynamic range. Set the ISO to 400 (still without HTP) and even more analog amplification is applied before the ADC digitizes the signal.

There is never a case where the starting point with ISO 50 or ISO 200 + HTP is any different from the exact same shot (same aperture and shutter) at ISO 100. The difference is entirely in the value displayed for the camera's meter and the way the raw file is processed. Any time you shoot with ISO 50 or ISO 200 + HTP, you can do the exact same thing by using the same aperture and shutter as you would at the expanded ISO settings but using ISO 100 instead, and then doing your own digital push or pull in post-processing.

So, why would you want to use either?

If you're shooting JPEGs, ISO 50 is useful when you wish you had a one-stop neutral density filter but you don't.

If you're shooting JPEGs and you care more about the highlights than the shadows -- such as when photographing a bride in a white dress -- HTP will cause the JPEG to render those highlights with more visible detail.

If you're shooting raw and doing ETTR (expose-to-the-right), you're doing the exact same thing as ISO 50. So, you might as well set the camera to ISO 50 and thereby get a preview image on the back of the camera that's closer to your desired final rendering.

If you're shooting raw and you're doing ETTL (expose-to-the-left) in order to capture as much bright detail as possible, you again might as well turn on HTP again to get a more accurate preview.

Lastly, if you're doing a very methodical manual HDR shoot, you might want to consider using ISO 50 for the shadow exposure and HTP for the highlight exposure -- being careful to actually properly adjust the shutter. That is, you might shoot, all at f/8, 1/200s @ ISO 50, 1/400s @ ISO 100, and 1/800s @ ISO 200 + HTP. All three images will be processed internally as if all were shot at ISO 100, but the JPEG previews for all three will be rendered to look very similar. However, the shadows will be cleanest with the ISO 50 shot and there will be more highlight detail with the ISO 200 shot. You could then process them identically in Photoshop and mask in the highlights and shadows from the respective files and get a seamless, natural-looking image with cleaner shadows and more highlight detail. You could, of course, do the exact same thing by shooting them all at ISO 100 and manually applying the digital exposure compensation before layering and masking them in Photoshop.

The data recorded by the sensor (and, presumably, written to the raw file) is identical for ISO 50 and ISO 100; all that's changed is the meter is told to overexpose by a stop and the raw processing engine is told to underexpose by a stop.

no they are notAt 100iso with the same metering the sensor charge is under 100 % and at the read out =before overload and clipping with 50iso your double the time or open up the lens 1 stop and therefore blow one stop of highlight.

I read that you stated that ISO 50 changes shutter speed or aperture by one stop. Do you believe that?

Neuroput your camera in front of a white wall, see what you get for values ​​at 100 iso then compare with 50isoyou will find that the exemplel 1/60sec f-5.6 will be with 50iso 1/60 sec f-4, 0 = 1 stop richer exposed compared to 100iso and you lose 1 stop of high light

I'm talking about the RAW file, not the metering. Unless I change the aperture or shutter, or let the camera do that (in an auto exposure mode like Av or Tv), there will be no difference in the sensor-derived image data between ISO 50 and ISO 100. Both are ISO 100 exposures, tthe ISO 50 data will merely be pulled down a stop by the RAW engine.

As for HTP, it's possible we're saying the same thing in different ways about the what (HTP ISO 200 is actually being underexposed at ISO 100 then brought back up). But where you seem to be wrong is the when - your contention is apparently that the data are altered on the sensor. Perhaps I misunderstand you, and you are referring to the actual ISO 100 exposure when ISO 200 is set in camera as that alteration. But what I'm saying, and TrumpetPower is saying, is that the HTP ISO 200 exposure RAW file is the same as an ISO 100 (non HTP) RAW file, and your statement that, "you halve the number of electrons" indicates you believe the RAW file is different. If you mean relative to an actual ISO 200 (non HTP) exposure, fine - but that's not what we've been talking about.

If you mean ISO 100 RAW is different than HTP ISO 200 RAW, that's wrong, and if you think that's what the page you linked and keep on re-quoting is saying, you're misinterpreting it. It states: "In post-processing, the image data can be brought back up while preserving the highlights with a modified tone curve...." Post-processing, not the RAW data file, as your statement about halving the number of electrons indicates. There is no change in the number of electrons between ISO 100 and HTP ISO 200, both are ISO 100 exposures, same number of electrons, just handled differently in post. If you don't understand that, perhaps you should re-read those links of yours.

The sensor does not care about iso, iso step is added after the readout.

Actually, that's not true -- and, presumably, at the heart of your misunderstanding.

Thanks for explaining Trumpetpower & Dr. Neuro (which I usually trust to be correct), htp & iso50 aren't self-explanatory and hardly documented anywhere - and in other articles there's still the theory that htp does some magic because it's done inside the image pipeline - probably because no one knows the exact tone curve Canon is applying.

"Halves the amount of light"? Please correct me if I am wrong..... But there is nothing you can do with sensor settings or camera modes that will change the amount of light.... You can play with gain and linearity and mapping color depths, but the amount of light remains unchanged. You can change the amount of light with shutter speed, aperture, or slapping on a neutral density filter, but not with the sensor.

Thanks for explaining Trumpetpower & Dr. Neuro (which I usually trust to be correct), htp & iso50 aren't self-explanatory and hardly documented anywhere - and in other articles there's still the theory that htp does some magic because it's done inside the image pipeline - probably because no one knows the exact tone curve Canon is applying.

Yep. Most RAW converters see the HTP flag in the metadata, and apply their version of Canon's tone curve. But some RAW converters ignore the metadata flag (e.g. Rawnalyze) and just show you the 1-stop underexposed image as it's actually recorded in the RAW image data.

"Halves the amount of light"? Please correct me if I am wrong..... But there is nothing you can do with sensor settings or camera modes that will change the amount of light.... You can play with gain and linearity and mapping color depths, but the amount of light remains unchanged. You can change the amount of light with shutter speed, aperture, or slapping on a neutral density filter, but not with the sensor.

that is not what Im saying, every iso step =is a halving of the number electroner, the sensor has no knowledge of iso at all, it collects photons and the number of photons / electrons is determined by time and the light inlet.

That's exactly what you said... "HTP. it is a halving of infaling light" are your exact words. Even this last statement from you is not even internally consistent.

that is not what Im saying, every iso step =is a halving of the number electroner, the sensor has no knowledge of iso at all, it collects photons and the number of photons / electrons is determined by time and the light inlet.

That is an absolutely incorrect statement.

With the now-beaten-to-death exceptions of ISO 50 and HTP, ISO is entirely a function of the sensor. As I've repeatedly attempted to explain, with increasing ISO the sensor applies increasing amounts of analog amplification, and all this happens on the sensor, well before the analog signal is digitized.

Yes, the number of photons that impinge upon the sensor is dependent upon the aperture and shutter (and, of course, the luminance of the scene as projected by the lens). But the number of electrons that reach the analog to digital converter (ADC) depends on how much analog amplification the sensor applies to the readout -- and the amount of amplification is directly set by the user (or the autoexposure system) with the ISO control.

With ISO 50, 100, and 200+HTP, the number of electrons per photon is the same. With ISO 200 (without HTP), thanks to analog amplification, twice as many electrons per photon make it to the ADC. The number of electrons per photon is doubled again with each additional stop of ISO.

(As a side note, "inbetween" ISO settings, those not powers of two times 100 or whatever the base ISO is, are again, with most camera systems, achieved by digital pushing or pulling from the nearest full-stop ISO. ISO 125 produces the same raw file as ISO 100 but with a metadata flag telling the raw processor to add 1/3 stop of digital push, and ISO 160 is really ISO 200 with 1/3 stop digital pull.)

And just, to further clarify what digital versus analog exposure adjustment means...if you were to write a computer program that translated the data in a raw file into a massive spreadsheet, divide every number in the spreadsheet by 2, and then translate from the spreadsheet back to the original raw file format, you'd do exactly the same thing that ISO 50 does. If you were to translate ISO 50, ISO 100, and ISO 200+HTP (assuming identical scenes, apertures, and shutter speeds) files each into separate spreadsheets, they'd all have the exact same numbers in them. But, if you exposed at ISO 200 (without HTP, but still keeping the shutter and aperture and everything else the same), the numbers in your spreadsheet would be twice as big...but they'd also have a higher standard deviation, indicating additional noise due to the distortion from the higher analog gain applied to the sensor readout. If you then, say, made the shutter a stop faster, the ISO 200 numbers would be back in line with the ISO 100 numbers, but you'd still have a higher standard deviation because of the additional noise from the increased analog amplification.

I really don't know how to express this any more clearly. If you still don't understand, then perhaps you should explain how you think the whole shebang actually functions, rather than just vaguely handwave about photons and electrons with unspecific and irrelevant references to Web pages that actually describe things correctly, and the opposite of what you describe.

HTP, here you have exposed the motive 1 stop shorter, halving the number of photons and you get a High light head room and then another curve is applied with a lift in lower areas/levels and a softer curve at the top / high lightsHere 100iso are exposed as i where 200iso which means 1 stop shorter exposure , the sensor collect less photons who are converted in to a charge/signal.

That's exactly what you said... "HTP. it is a halving of infaling light" are your exact words. Even this last statement from you is not even internally consistent.

Factual consistency does not seem to be one of Mikael's strong suits... But in other ways, he's quite consistent - his use of bold text, his derogatory questioning of others' understanding coupled with urging others to 'read' and 'try to understand', his repetition of the same statements in post after post, in these and similar areas, he seems consistent to the point of boredom.

With the now-beaten-to-death exceptions of ISO 50 and HTP, ISO is entirely a function of the sensor. As I've repeatedly attempted to explain, with increasing ISO the sensor applies increasing amounts of analog amplification, and all this happens on the sensor, well before the analog signal is digitized.

Yes and no, depends how you define the "sensor". It could be that this is a fine semantic point. There is a lot of circuitry on the same chip but does that make it part of the "sensor"? If the ADC were to be fully integrated onto the same chip is that part of the "sensor"? What about memory?

So, it is imperative then that we define what we mean by "sensor" and discuss based on a common definition. In my view, it is most practical to refer to the sensor as the "light/electron gathering" parts. Including much more than that gets right into the quagmire I described above.

The amount of light incident on the sensor is NEVER a function of ISO. It is only a function of aperture and shutter speed. ISO gain is applied after the exposure. The so-called analog gain is applied prior to readout but that is only up to some point (something like ISO 1600?) beyond which it is applied digitally after readout.

So, the statement that "ISO is entirely a function of the sensor" might be somewhat correct depending on how we define "sensor" but only up to about ISO 1600.

Factual consistency does not seem to be one of Mikael's strong suits... But in other ways, he's quite consistent - his use of bold text, his derogatory questioning of others' understanding coupled with urging others to 'read' and 'try to understand', his repetition of the same statements in post after post, in these and similar areas, he seems consistent to the point of boredom.

Because the readout is the point where the signal logically leaves the sensor, it should be self-apparent that analog gain must be applied by the sensor itself to the signal before (or as) it is read out.

You're of course correct that expanded high ISOs are again digital simulations and not analog processes, but I suspect your cutoff of ISO 1600 only applies to very old cameras. I may be mistraken, but I'm pretty sure that digital ISO boost only happens with expanded ISO settings. With the 5DIII, native (analog) ISO should extend all the way to ISO 25,600...but I wouldn't bet more than a cup of coffee on that exact number.

"Halves the amount of light"? Please correct me if I am wrong..... But there is nothing you can do with sensor settings or camera modes that will change the amount of light.... You can play with gain and linearity and mapping color depths, but the amount of light remains unchanged. You can change the amount of light with shutter speed, aperture, or slapping on a neutral density filter, but not with the sensor.

Obviously a number of us are clueless. HTP stands for Half The Photons! Now how that happens in the sensor needs to be discovered...

And oh, just in case somebody takes my post at face value, here is the tag:<SARCASM/>

Because the readout is the point where the signal logically leaves the sensor, it should be self-apparent that analog gain must be applied by the sensor itself to the signal before (or as) it is read out.

Depends when, where, and how that "analog gain" is being applied and on what we think is the beginning of the read out process. My suggestion is that we think of the "sensor" as the photo-sensitive material and the electron wells. Everything after that is electronics and should be considered as part of the read-out circuitry including the analog gain. Applying the analog gain is not part of the exposure per se and, it adds noise which by the usual definition is part of the read-out noise (if we agree on two categories of noise those being photon shot-noise and read-out noise).